This invention relates generally to semiconductor fabrication techniques and, more particularly, to masking techniques.
As it is known in the art, a semiconductor circuit is fabricated by using sequences of semiconductor growth, etching, metalization, and patterning steps, which are used in various combinations to form the desired semiconductor circuit or integrated circuit. One of the most commonly used steps in semiconductor fabrication process are masking steps. Generally the masking steps include the use of a resist which is patterned either by projecting a desired image through a mask onto the resist layer or disposing a mask on the resist layer and imaging through the mask. The image is used to selectively sensitize regions of the resist material.
Resists generally are of several types. The first classification for a resist is the classification pertaining to the portion of the electromagnetic spectrum used to sensitize the resist. In general, resists are a three component system. The first component is a resin, such as a phenolic resin which forms a matrix when deposited and a chemical activator which is sensitive to particular electromagnetic energy and a solvent in which the other two materials are dissolved. The solvent is generally driven off prior to imaging the resist and is used as a vehicle to apply the solids in the resist to the surface to be masked. Resists are available which are sensitive to various bands of electromagnetic energy including resists sensitive to exray energy, electron beams, as well as photons of optical energy in the UV, as well as, optical portions of the electromagnetic spectrum. A principal factor used in selecting a resist is the degree of resolution required for the formed photoresist pattern.
The second classification for a resist are resists which are positive resists and resists which are negative resists. A positive resist is a resist having a positive electromagnetic energy activator which when exposed to such energy undergoes a chemical change with the resin matrix material providing an exposed resist region which can be washed away in a developing process by use of a suitable developer. On the other hand, a negative photoresist undergoes the exact opposite transformation, that is, providing an activator chemical which undergoes a chemical transition causing the exposed portion of the resist to be inert to the developing chemical.
Other types of resists such as polymethylmethacrylate develop a latent image through fissioning of polymer chains as a result of incident high energy electron beams or energetic photons.
Within each photoresist type particularly positive photoresist which are activated by electron beams or optical photons (i.e. either deep UV or UV), there are many different commercially available photoresists. The particular photoresists chosen is chosen for various properties including its degree of adhesion to materials over which they are disposed, its chemical and thermal resistance to solvents used during other portions of a semiconductor fabrication process and as mentioned above its optical imaging characteristics. Thus, while each of the aforementioned properties of resist are important, a particular semiconductor fabrication step may dictate the importance of one property over another. Often, a resist chosen for imaging qualities has concomitantly poor adhesion properties, particularly on semiconductor materials.
As an illustrative example of the aforementioned problem, a photoresist generally denoted as PMMA (polymethylmethacrylate) is commonly used on gallium arsenide for providing patterns having a high degree of resolution, as for example, the patterns which are commonly used during steps of forming gate electrodes for field effect transistors fabricated on the gallium arsenide. PMMA is a popular resist patterning material for submicron gates on gallium arsenide wafers because of its excellent optical imaging properties. However, as described in a paper entitled "Gallium Arsenide Process Compatibility Adhesion for PMMA Resists" by Levinson, et al., Electro Chemical Society, Solid State Science and Technology, Vol. 133, No. 3 (1986), page 619, PMMA is prone to undercutting and adhesion failure particularly during wet chemical etching processes.
Proper adhesion of the PMMA resist to the gallium arsenide is a critical requirement since adhesion failure will cause a failure in the etching step. In a typical process fabrication flow for such devices, the gate deposition step is one of the final steps in the processing of a wafer including such semiconductor devices. Failure at that juncture is extremely costly and troublesome. One solution suggested for this problem is an adhesion promoter referred to as hexamethyldisilazane. This material is an adhesion promoter for silicon and surfaces containing silicon. The promoter acts to form a siloxane bond with the silicon containing surface. However, it has been reported in "Gallium Arsenide Processing Techniques" by Williams, et al., Art Tech House, Inc. (1984), p. 137, that hexamethyldisilazane is not an effective adhesion promoter without the formation of siloxane bonds. Therefore, hexamethyldisilazane is probably not an effective promoter for use with gallium arsenide since such siloxane bonds will not form.
Another approach to this problem has been to bake (i.e. cure) the deposited resist at an elevated temperature over an extended period of time than otherwise used. While this has some effect in increasing adhesion promotion, it also has the drawback of reducing the ability of a developer to remove exposed portions of the resist. This occurs because the extended baking of the phenolic resin causes the resin to undergo a chemical reaction. That is, the resin material which is in the form of a long chain polymer undergoes cross linking polymerization. This is very undesirable for effective removable of exposed portions of the photoresist since cross linking increasing the integrity and the resistance of the material to removal during developing of the resist. This is undesirable for providing patterns having good resolution.